(1) Field of the Invention
The present invention relates generally to determining the transfer function of a fluid-filled compliant cylinders and more particularly to a field method by which the calibration of the transfer function of a fluid-filled compliant cylinder is measured as a function of temporal frequency and spatial frequency (or wavenumber).
(2) Description of the Prior Art
Towed acoustic arrays are used in a variety of commercial and military applications. For example, towed arrays are used in seismic survey applications as well as in antisubmarine warfare applications. In general a towed array is used to measure a pressure field propagating in a fluid environment.
A towed array typically consists of a compliant (e.g., rubber or plastic) fluid-filled hose or cylinder containing an array of hydrophones or other sensors for making measurements of the pressure field incident upon its outer surface. The pressure field of interest will be referred to hereinafter as the signal. When there is relative movement between the fluid environment and the cylinder (e.g., the cylinder is towed or the cylinder is stationary and the fluid environment is moving), a turbulent pressure field develops which also impinges on the outer surface of the cylinder. This pressure field will be referred to hereinafter as the noise pressure field.
The major elements of the noise pressure field are separated in spatial frequency or wavenumber from the signal and can be filtered by a spatial filter. The cylinder acts as the first and primary spatial filter to the noise pressure field arising from the turbulent boundary layer. As such, it is desirable to be able to calibrate or measure the attenuation provided by the cylinder transfer function T.sub.H (k,.omega.) as a function of the spatial frequency or wavenumber k and the frequency .omega.. The cylinder transfer function T.sub.H (k,.omega.) is defined as the ratio of inner pressure to outer pressure according to equation (1) EQU T.sub.H (k,.omega.)=P.sub.i (k,.omega.)/P.sub.o (k,.omega.)(1)
where P.sub.i (k,.omega.) is the pressure field measured in the fluid in the cylinder at a radius r and P.sub.o (k,.omega.) is the pressure field impressed on the outer surface of the cylinder. At present, no impressed on the outer surface of the cylinder. At present, no methods exist to field calibrate the cylinder transfer function T.sub.H (k,.omega.) in which pressure fields P.sub.i (k,.omega.) and P.sub.o (k,.omega.) are measured.